The staff of Clinical Studies Section in Boston, Massachusetts has sifted a large volume of clinical data to identify patients for etiologic studies. In the last 4 years, over 600 questionnaire interviews of childhood-cancer patients have been completed. The data have been analyzed, and a report in preparation. Through this and other mechanisms of ascertainment, several unusual familial aggregates of cancer have been identified: familial mesothelioma after asbestos exposure; X-linked familial leukemia and pre-leukemia; a familial syndrome of acute leukemia, cerebellar ataxia, and chromosome deletion; Turcot's syndrome in 3 siblings; Wilms' tumor in 5 cousins; and autosomal dominant renal cell carcinoma associated with a constitutional chromosome translocation. A computerized Registry of Survivors of Childhood Cancer has been established through collaboration of the Clinical Studies Section with the Sidney Farber Cancer Institute. The 500 registrants represent approximately 2 percent of all survivors of U.S. childhood cancer in the last 2 decades. Patients in the series are being studied for second malignancies, cancer in close relatives, and genetic effects in progeny. A death certificate study of lung cancer in selected Maine counties with high mortality rates has been completed, and field interviews are starting.